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What Is This Switch In Avionic Bay ?

Avionic bay is a section under the cockpits of aircraft where system boxes, circuit breakers and batteries are located. Some deactivation / reactivation and troubleshooting procedures are performed from this section.

Apart from circuit breakers and boxes, there is also a switch in this compartment. Let’s take a look at the function of switch.


Due to some failures and maintenance operations, aircraft may need to fly with the landing gear down. To explain this with a few examples; with the covid-19 pandemic that has been going on since 2019, many companies have storaged some of the aircraft in their fleet.

According to the storage procedure, landing gear retraction/extension and freefall extension tests need to be performed in return-to-service applications of an aircraft that has been storaged for more than 12 weeks (It may be different depending on the aircraft types.).


Flight With Landing Gear Down

Because of an airport that does not have enough equipment (Jacks, external hydraulic power device) to perform these tests, storaged aircraft fly to a better equipped airport with the landing gear down and the landing gear doors closed.

This procedure is applied since it cannot be determined whether there is a failure in the landing gear because the test cannot be performed. Also some failures that prevent the landing gear operation are examples for this situation. (eg turn-buckle link breaking)

According to the flight with landing gear down procedure in ATA Chapter 5 Special flights” tab, these ALTERNATE VMO/MMO SWITCH (Boeing) or L / G DOWN VMO / MMO SELECTION (Airbus) are set to ON position in E&E compartment.


ADIRU provides attitude,heading and navigation data for aircraft systems by sensing from pitot tubes, static ports, total air temperature probes and angle of attack sensors.

FOR B737

When the VMO/MMO Switch is set to ON position, the ADIRU adjust to Vmo/Mmo curve to compansate for a flight with the landing gear down.

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